1. Field of the Invention
This invention relates in general to the alignment of liquid crystals in electro-optical devices, and more particularly to a stable tilted alignment in liquid crystal light valves and displays for low voltage operation.
2. Description of the Related Art
Photoactivated liquid crystal light valves convert an input image to an electric field pattern across a liquid crystal cell, whereby the liquid crystal material within the cell is rotated in accordance with the local field. The liquid crystal orientation controls the transmission of an output beam from the light valve such that the output beam contains spatial information from the input beam, but often in a different format. One of the problems encountered with these devices is that, when the liquid crystals are initially aligned with their directors perpendicular to the cell walls in the absence of an applied field, the direction in which they tilt when a field is applied is somewhat random. Liquid crystals in different areas of the cell may tilt in different directions; this phenomenon can interfere with proper light valve operation, especially in the presence of lateral fields resulting from lateral gradients in the input light intensity.
In an attempt to overcome this problem, liquid crystal cells have been fabricated in which the directors of the liquid crystals are given a slight but uniform pretilt in the absence of an electric field. Thus, when a field is applied, the crystals will tilt in the same direction because of their uniform pre-tilt This technique has achieved good short term results in improving light valve operation, but thus far has suffered from a lack of adequate photostability. When exposed to light for significant periods of time, the tilted alignment deteriorates and the crystals return to a perpendicular alignment in the absence of a field. Thus, while a tilted alignment has been demonstrated to improve light valve operation, the short lifetime of devices which employ this technique has placed a practical limitation on their usefulness.
The desirability of maintaining a liquid crystal pretilt of about 2.degree. to about 6.degree. for a light valve is described in an article by Grinberg, et al., "Photoactivated Birefringent Liquid-Crystal Light Valve for Color Symbology Display", IEEE Transactions on Electron Devices, Vol. ED-22, page 775 (1975). This paper also cites the problem of liquid crystal deterioration from ultraviolet and visible light of high intensity.
A prior art technique for achieving a tilted liquid crystal alignment is described in U.S. Pat. No. 4,030,997 by Leroy J. Miller and Jan Grinberg, issued June 27, 1977 and assigned to Hughes Aircraft Company, the assignee of the present invention. This patent describes a technique in which the surface of a substrate is overcoated with a thin layer of SiO.sub.2 or other material with reactive surface hydroxyl groups, and then ion beam etched at a shallow angle. The surface is next treated with a mixture of a long chain aliphatic alcohol having the formula CH.sub.3 (CH.sub.2).sub.n CH.sub.2 OH, where n ranges from about 2 to about 18, and an aliphatic amine catalyst The treatment takes place by immersing the substrate in a hot liquid mixture of the alcohol and amine, followed by washing the surface with an inert organic solvent. In this approach the alcohol reacts to convert SiO.sub.2 surface hydroxyl groups into alkoxy groups. The liquid crystal tilt angle has been found to be dependent upon the ion beam etching conditions, the length of the alkoxy group bonded to the surface, the liquid crystal mixture used in the cell, and the temperature. Typical photostability lifetimes of cells formed by this method were only about two to four Wh/cm.sup.2 of exposure from a xenon arc lamp filtered to give a broad band exposure (385-950 nm) with an intensity of about 150 mW/cm.sup.2 on the light valve cell. These cells failed rapidly because the off-perpendicular liquid crystal tilt was rapidly and excessively reduced, e.g., a cell change from an initial tilt of 3.degree. down to 0.4.degree. after only three Wh/cm.sup.2 of exposure. The initial tilt angle of the cells also varied substantially with the cell temperature.
Several reports have been made on other techniques for obtaining tilted-perpendicular liquid crystal surface alignment, but none of them disclosed any significant improvement in the photostability of the pre-tilt alignment. An article by W. Urbach et al. in Applied Physics Letters, Volume 25, page 479 (1974) describes the use of the surfactant CTAB (cetyltrimethylammonium bromide) on angle-evaporated coatings of SiO to obtain perpendicular or tilted-perpendicular alignment. A CTAB coating is achieved by slowly pulling the plates vertically from the surfactant solution. The CTAB coating on a shallow angled SiO deposition, deposited at a grazing angle of about 15.degree., gave large off-perpendicular liquid crystal alignment. Small off-perpendicular alignments were achieved for a medium angle SiO deposition deposited at a grazing angle of about 25.degree., while a perpendicular alignment resulted from SiO deposited at a large grazing angle of about 50.degree.. Because the CTAB is not chemically bonded to the surface, some or even all of it can dissolve in the liquid crystal, depending upon the liquid crystal structure, thickness and temperature. The dissolved CTAB will increase the conductivity of the liquid crystal, which is undesirable for a field-effect device such as tuneable birefringence. Neither the surface nor the dissolved CTAB would be expected to have long term photostability at high radiation intensities.
The use of lecithin or "acid T" as a surfactant dopant for bulk liquid crystals in cells with medium angle deposited SiO (15.degree.-30.degree. off-surface deposition), or with medium angle deposited MgF.sub.2 treated surfaces, is described in K. Fahrenschon and M. F. Schiekel, Journal of the Electrochemistry Society, Vol. 124, page 953 (1977). The article gives examples of 12.degree. and 16.degree. pre-tilts for the liquid crystals in the tilted-perpendicular cells. The surfactants used are not chemically bonded to the surface, and have not been found to be either thermally or photochemically stable alignment agents, especially for long period, high intensity exposures.
W. R. Heffner, et al., Applied Physics Letters, Vol. 36, page 144 (1980) described the use of silane (DMOAP) and of ultrathin plasma polymerized tetrafluoroethylene (UTPFE) on obliquely deposited silicon monoxide shallow angle deposition-SiO.sub.x surfaces, with a 5.degree. grazing angle, to obtain tilted-perpendicular liquid crystal alignment. Off-normal liquid crystal angles of 16.degree.-20.degree. were reported with the silane/SiO treatment and the liquid crystal CB-7, 20.degree.-35.degree. with an azoxy liquid crystal, and 22.degree.-32.degree. with the UTPFE/SiO treatment and CB-7. Each of these "surfactant" reagents are reported to give well adhered polymer coatings on glass (or SiO) surfaces, and these could be considerably more stable than the CTAB and lecithin surfactants reported above. However, no devices were described and no photostability data were reported.
An article by L. Rousille and J. Robert in Journal of Applied Physics, Vol. 50, page 3975 (1979) describes the use of plasma polymerized polytetrafluoroethylene (PTFE) films 20 angstroms thick on medium angle deposited-SiO (30.degree. grazing angle deposition) surfaces. Liquid crystals with about a 3.degree. off-perpendicular tilt were obtained. Thicker PTFE films of 50 angstroms resulted in 0.degree. tilt on the shallow angle deposited-SiO. No photostability or thermal stability lifetime data were reported or discussed.
Techniques for obtaining surface-perpendicular (nontilted) liquid crystal alignment by forming surface alkoxy groups bonded to the surface by the reaction of long chain alcohols with surface hydroxyl groups are described in U.S. Pat. No. 4,022,934 by Leroy J. Miller, issued May 10, 1977 and U.S. Pat. No. 4,464,134 by Anna M. Lackner et al., issued Aug. 7, 1984, both assigned to Hughes Aircraft Company. The Miller patent describes the reaction of the surface with long chain alcohols by immersing the surface in a hot bath of the alcohol or an alcohol/ amine mixture. The Lackner et al. patent describes an improved technique of reacting the surfaces with long chain alcohols by exposure to hot alcohol vapor. The two patents deal with the achievement of a perpendicular alignment, rather than a tilted alignment. The Lackner et al. patent deals with methods for obtaining substantially perpendicular liquid crystal surface alignment on surfaces which include an oxide coating selected from the group consisting of SiO.sub.2, SiO, indium-tin-oxide/SiO.sub.2, tin-oxide/ SiO.sub.2 and indium/SiO.sub.2 mixtures.
Another technique that is relevant to the present invention is reported in an article by Milo Johnson and P. Andrew Penz, "Low Tilt Angle Nematic Alignment Compatible With FRIT Sealing", IEEE Transactions on Electron Devices, Vol. ED-24, No. 7, July 1977, pages 805-807. The reported technique is used for tilted surface-parallel liquid crystal alignment. In this technique a combination of medium angle deposition (MAD) and shallow angle deposition (SAD) of SiO.sub.x on a surface with a basically planar macrostructure is used to achieve a tilted surface microstructure having a generally saw-tooth-shaped profile. This technique is used for low tilt homogenous liquid crystal alignment.
Despite the considerable amount of work that has been done in both the treatment of surfaces to obtain tilted microstructures, and in methods for obtaining both tilted and perpendicular liquid crystal orientations, a liquid crystal cell capable of retaining a tilted-perpendicular liquid crystal orientation over extended operating periods in a light valve was not achieved until the subject matter of related Patent Application Ser. No. 919,155 filed Oct. 14, 1986, was deVeloped.
In the related patent application, one or both opposed surfaces of a liquid crystal cell are treated so as to provide the surfaces with generally aligned microstructures having tilt angles relative to the surface macrostructures which may be inferred to fall within the range of about 0.5.degree. to about 6.degree.. The surfaces are provided with free hydroxyl groups. The desired microstructure configuration is achieved by MAD (medium angle deposition) and SAD (shallow angle deposition) depositions of SiO.sub.x, where x is from one to two, with the two depositions oriented approximately 90.degree. to each other. A long chain alcohol is then reacted with the surface and alkoxy groups are chemically bonded to the tilted surface microstructures, the alcohol molecules having the formula ROH, where R is a carbon chain having from about 6 to about 24 carbon atoms. The alcohol may be mixed with an amine, and is preferably bonded to the tilted surface microstructure by exposing the surface to the alcohol vapor. The substrate itself is preferably formed from SiO.sub.x. Negative dielectric anisotropy liquid crystals introduced into the prepared cell are aligned by the alkoxy groups to achieve the desired zero-field tilt.
While cells formed in this manner have been found to exhibit a much higher degree of tilt photostability than in previous techniques, both their temperature range and speed of response have been found to be somewhat limited by the availability of suitable liquid crystals of negative dielectric anisotropy. Also, a faint honeycomb pattern known as the "chicken wire effect" has been noticed in the output of some photoactivated liquid crystal light valves. This is believed to arise from the practice of driving the light valve with light from a cathode ray tube through a fiber optic face plate, in which the optical fibers are not perfect and produce the undesired chicken wire effect. The liquid crystals are selected from negative dielectric anisotropy materials, meaning that they rotate away from the field established across the cell in response to an applied input image.